1. Field of the Invention
The present invention relates to an electronic circuit for generating a trip signal to operate a circuit interrupter in a line carrying alternating current and, more specifically, to a circuit for generating a trip signal to initiate the interruption of overcurrent in a line in response to, and a predetermined time after, the inception of overcurrent in the line. The predetermined time is inversely related to the magnitude of the overcurrent.
2. Description of the Prior Art
It is well known that typical prior art circuit interrupters such as fuses, circuit switchers, circuit breakers, and reclosers may respond to certain overcurrents according to various time-current characteristics. Some interrupters rely upon the effect of the overcurrent on the interrupter to open the circuit. For example in a fuse, overcurrent melts a fusible element to open the circuit, with the magnitude of the overcurrent determining how quickly the fusible element melts, thereby providing an inverse time-current characteristic. Other devices have used various types of sensors to detect current in a line and to operate circuit interrupters in response to overcurrent.
Prior art circuit interrupters operate according to time-current relationships which are characteristic of each particular type of interrupter. The time-current characteristic of many prior art circuit have provided limited flexibility in constructing a circuit interrupter having a particular overall time-current characteristic. For example, it would be desirable to match the time-current characteristic of an interrupter as precisely as possible to the damage curve of a transformer or other piece of equipment to be protected by the interrupter. Thus, it would be desirable in a circuit interrupter to be able to select a time-current characteristic with both straight and arcuate portions.
It is desirable that the higher the level of overcurrent in a protected line, the more quickly a circuit interrupter operates. Further, it is desirable that such circuit interrupters not respond to current in the line that is not overcurrent, regardless of the length of time such lower magnitude current flows.
It is desirable that circuit interrupters be operated reliably whenever overcurrent exists. It is also desirable that the level of current to which, and the delay before, the circuit interrupters responds be appropriate in light of the physical characterstics of the circuit interrupter and of the equipment in the line being protected.
Accordingly, one object of the present invention is to provide a trip signal generator for generating a trip signal to reliably operate an associated circuit interrupter.
It is a further object of the present invention to provide a trip signal generator for producing a trip signal after a time delay has elapsed following the onset of an overcurrent, the length of such time delay being inversely related to the magnitude of the overcurrent.
It is also an object of the present invention to provide a trip signal generator that can be readily adapted to respond to various levels of overcurrent, after various time delays, thereby permitting the trip signal generator to be matched with the design limitations of the associated circuit interrupter and of the equipment in the line being protected.
It is another object of the present invention to provide a trip signal generator in which the period of delay before a trip signal is generated in response to a given level of overcurrent is readily selectable, thereby permitting further adjustment of the trip signal generator to suit the design limitations of the associated circuit interrupter and of the equipment in the line.
Yet another object of the present invention is to provide a versatile trip signal generator that can be used to control the operation of various types of circuit interrupters and other devices, including, for example, circuit breakers, reclosers, circuit switchers, alarms, and the like.